Hollow-porous fibers for intrinsically thermally insulating textiles and wearable electronics with ultrahigh working sensitivity

Room-temperature and recyclable preparation of cellulose nanofibers using deep eutectic solvents for multifunctional sensor applications

Cellulose nanofibers (CNFs) have gained increasing attention due to their robust mechanical properties, favorable biocompatibility, and facile surface modification. However, green and recyclable CNF production remains challenging. Herein, a green, low-cost and room-temperature strategy was developed to exfoliate CNFs using deep eutectic solvents. A high average yield (~90 %) of CNFs was achieved during the recycling process. The resultant CNFs delivered favorable dispersion in water with an average length of ~5.3 μm and an average diameter of ~44 nm. The application of the resultant CNFs for multifunctional sensors was explored by fabricating the composite films of poly(vinyl alcohol) (PVA), tannic acid-decorated CNFs (CNF@TA) and Ag nanoparticles (AgNPs). As a stretchable strain sensor, the PVA/CNF@TA/AgNPs sample exhibited superior sensitivity (GF = 46.42), low detection limit (<1 %) and fast response (80 ms). This sensor possessed excellent temperature sensing performance with good accuracy (0.1 °C), high TCR (29.84/°C) in the body temperature region of 34-42 °C and desirable linearity (R2 = 0.981). In addition, the sensor could be used to monitor real-time skin moisture information. This simple and economical preparation strategy may facilitate potential applications of CNFs in the development of multifunctional sensors for wearable electronic devices.

A flexible multifunctional sensor based on in situ reduction of Ag nanoparticles by yam polysaccharides

Developing flexible multifunctional sensors that combine humidity, temperature, and strain sensing properties is a challenge. In this paper, PVA/YPs/H3PO4/AgNPs (PYHA) flexible composite films loaded with Ag nanoparticles (AgNPs) were synthesized through in situ reduction and solution casting using polyvinyl alcohol (PVA), yam polysaccharide (YPs), phosphoric acid (H3PO4), and silver nitrate (AgNO3) as raw materials, which exhibited sensitivity to humidity, temperature, and strain. The prepared PYHA humidity sensor was capable of generating stable electrical signals through adsorption and desorption over the relative humidity (RH) range of 35-95 %. Furthermore, the humidity sensor displayed minimal hysteresis (2.17 % RH) and excellent linearity (R2 = 0.973) during respiratory rate monitoring in different body states. As a temperature sensor, the PYHA sensor was capable of sensing human body temperature, exhibiting strong temperature sensitivity (TCR = -1.058 % °C-1) and maintaining excellent linearity (R2 = 0.994) ranging from 35 to 95 °C in temperature. Moreover, the PYHA flexible strain sensor boasted an extensive strain detection range (1-320 %) and swift response/reply time (0.8/1.1 s), and detected physiological signals due to large movements of body joints and weak changes in facial expressions. Therefore, the designed PYHA multifunctional sensor has a promising use in flexible wearable.

A high stretchability fiber based on a synergistic three-dimensional conductive network for wide-range strain sensing

Fiber strain sensors are promising for constructing high-performance wearable electronic devices due to their light weight, high flexibility and excellent integration. However, the conductivity of most reported fiber strain sensors is severely degraded, following deformation upon stretching, and it is still a considerable challenge to achieve both high conductivity and stretchability. Herein, we have fabricated a fiber strain sensor with high conductivity and stretchability by integrating the AgNPs into the multi-walled carbon nanotube/graphene/thermoplastic polyurethane (MWCNT/GE/TPU) fiber. The tunneling-effect dominated MWCNT/GE layer bridges separated AgNP islands, endowing conductive fibers with the integrity of conductive pathways under large strain. By means of the synergistic effect of a three-dimensional conductive network, the fiber strain sensor of AgNPs/MWCNT/GE/TPU presents not only a high conductivity of 116 S m-1, but also a wide working range of up to 600% and excellent durability (8000 stretching-releasing cycles). Remarkably, benefiting from the crack propagation on the brittle AgNP layer, the fiber strain sensor exhibits a large resistance change in the strain range of 500-600%, and thus high sensitivity with a gauge factor of 545. This fiber strain sensor can monitor human physiological signals and body movement in real-time, including pulse and joint bending, which will contribute to the development of smart textiles and next-generation wearable devices.

Topological Polymer Networks-Enabled Mechanically Strong Polyamide-Imide Aerogel Fibers for Thermal Insulation in Harsh Environments

Aerogel fibers have sparked substantial interest as attractive candidates for thermal insulation materials. Developing aerogel fibers with the desired porous structure, good knittability, flame retardancy, and high- and low-temperature resistance is of great significance for practical applications; however, that is very challenging, especially by using an efficient method. Herein, mechanically strong and flexible aerogel fibers with remarkable thermal insulation performance are reported, which are achieved by constructing stiff-soft topological polymer networks and a multilevel hollow porous structure. The combination of polyamide-imide (PAI) with stiff chains and polyurethane (PU) with soft chains is first found to be able to form a topological entanglement architecture. More importantly, multilevel hollow pores can be constructed synchronously through just a one-step and green wet-spinning process. The resultant PAI/PU@340 aerogel fibers show an ultrahigh breaking strength of 94.5 MPa and superelastic property with a breaking strain of 20%. Furthermore, they can be knitted into fabrics with a low thermal conductivity of 25 mW/(m·K) and exhibit attractive thermal insulation property under extremely high (300 °C) and low temperatures (-191 °C), implying them as promising candidates for next-generation thermal insulation materials.

Research on high sensitivity piezoresistive sensor based on structural design

With the popularity of smart terminals, wearable electronic devices have shown great market prospects, especially high-sensitivity pressure sensors, which can monitor micro-stimuli and high-precision dynamic external stimuli, and will have an important impact on future functional development. Compressible flexible sensors have attracted wide attention due to their simple sensing mechanism and the advantages of light weight and convenience. Sensors with high sensitivity are very sensitive to pressure and can detect resistance/current changes under pressure, which has been widely studied. On this basis, this review focuses on analyzing the performance impact of device structure design strategies on high sensitivity pressure sensors. The design of structures can be divided into interface microstructures and three-dimensional framework structures. The preparation methods of various structures are introduced in detail, and the current research status and future development challenges are summarized.

Progress in microwave absorbing materials: A critical review

Microwave-absorbing materials play a significant role in various applications that involve the attenuation of electromagnetic radiation. This critical review article provides an overview of the progress made in the development and understanding of microwave-absorbing materials. The interaction between electromagnetic radiation and absorbing materials is explained, with a focus on phenomena such as multiple reflections, scattering, and polarizations. Additionally, types of losses that affect the performance of microwave absorbers are also discussed, including dielectric loss, conduction loss, relaxation loss, magnetic loss, and morphological loss. Each of these losses has different implications for the effectiveness of microwave absorbers. Further, a detailed review is presented on various types of microwave absorbing materials, including carbonaceous materials, conducting polymers, magnetic materials, metals and their composites, 2D materials (such as MXenes and 2D-transition metal dichalcogenides), biomass-derived materials, carbides, sulphides, phosphides, high entropy (HE) materials and metamaterials. The characteristics, advantages, and limitations of each material are examined. Overall, this review article highlights the progress achieved in the field of microwave-absorbing materials. It underlines the importance of optimizing different types of losses to enhance the performance of microwave absorbers. The review also recognizes the potential of emerging materials, such as 2D materials and high entropy materials, in further advancing microwave-absorbing properties.

Recent Advances in Flexible Wearable Technology: From Textile Fibers to Devices

Smart textile fabrics have been widely investigated and used in flexible wearable electronics because of their unique structure, flexibility and breathability, which are highly desirable with integrated multifunctionality. Recent years have witnessed the rapid development of textile fiber-based flexible wearable devices. However, the pristine textile fibers still can't meet the high standards for practical flexible wearable devices, which calls for the development of some effective modification strategies. In this review, we summarize the recent advances in the flexible wearable devices based on the textile fibers, putting special emphasis on the design and modifications of textile fibers. In addition, the applications of textile fibers in various fields and the critical role of textile fibers are also systematically discussed, which include the supercapacitors, sensors, triboelectric nanogenerators, thermoelectrics, and other self-powered electronic devices. Finally, the main challenges that should be overcome and some effective solutions are also manifested, which will guide the future development of more effective textile fiber-based flexible wearable devices.

Achieving a Wide-Range Linear Piezoresistive Response in Electrowritten Soft-Hard Polymer Blends via Salami-Inspired Heterostructure Design

Flexible electronics capable of acquiring high-precision signals are in great demand for the development of the internet of things and intelligent artificial. However, it is currently a challenge to simultaneously achieve high signal linearity and sensitivity for stretchable resistive sensors over a wide strain range toward advanced application scenarios requiring high signal accuracy, e.g., sophisticated physiological signal discrimination and displacement measurement. Herein, a film strain sensor, which has an electrical and mechanical dual heterostructure, was fabricated via a direct near-field electrowriting and molecule-guided in situ growth of silver nanoparticles with different concentrations on high-modulus polystyrene domains and low-modulus styrene-butadiene copolymers with a salami-like morphology. Mechanism analyses from both theoretical and experimental investigations reveal that the salami-like heteromodulus microstructure regulates microcrack propagation routes, while the heteroconductivity changes the electron transport paths and amplifies the resistance increase during crack propagation. Therefore, the as-designed strain sensor shows a linear resistive response within ca. 70% strain with a gauge factor of 25, unveiling a simple and scalable strategy for trading off signal linearity and sensitivity over a wide strain range for the fabrication of high-performance linear strain sensors.

In situ reduction of Ag nanoparticles using okra polysaccharides for the preparation of flexible multifunctional sensors

This paper reports the fabrication of flexible films loaded with Ag nanoparticles (Ag NPs) and annotated as POPA films from polyvinyl alcohol, okra polysaccharides, phytic acid, and AgNO3 via an in situ reduction and solution-casting method. The prepared films exhibit strain, temperature, and humidity sensing. As a flexible strain sensor, the POPA sensor has a wide strain sensing range (1-250 %), and fast response/recovery (0.22/0.28 s), while as a temperature sensor, it senses the human body temperature and exhibits excellent temperature sensitivity (TCR = -1.401 % °C-1) and good linearity (R2 = 0.994) in the temperature range of 30-55 °C. Additionally, in the relative humidity (RH) of range 35-95 %, the POPA humidity sensor outputs stable electrical signals during adsorption and desorption. Moreover, it exhibits low hysteresis values (3.19 % RH) and good linearity (R2 = 0.989) for the detection of breathing rates during different human body states. Consequently, the POPA sensor exhibits good stability, repeatability, and reversibility for strain, temperature, and humidity sensing. The designed multifunctional POPA sensor thus holds great potential for its application in flexible wearable devices and electronics.

Programmable and Weldable Superelastic EGaIn/TPU Composite Fiber by Wet Spinning for Flexible Electronics

As an essential component of flexible electronics, superelastic conductive fibers with good mechanical and electrical properties have drawn significant attention, especially in their preparation. In this study, we prepared a superelastic conductive fiber composed of eutectic gallium-indium (EGaIn) and thermoplastic polyurethane (TPU) by simple wet spinning. The composite conductive fiber with a liquid metal (LM) content of 85 wt % achieved a maximum strain at a break of 659.2%, and after the conductive pathway in the porous structure of the composite fibers was fully activated, high conductivity (1.2 × 105 S/m) was achieved with 95 wt % LM by mechanical sintering and training processes. The prepared conductive fibers exhibited a stable resistive response as the fibers were strained and could be sewn into fabrics and used as wearable strain sensors to monitor various human motions. These conductive fibers can be molded into helical by heating, and they have excellent electrical properties at a maximum mechanical strain of 3400% (resistance change <0.27%) with a helical index of 11. Moreover, the conductive fibers can be welded to various two or three-dimensional conductors. In summary, with a scalable manufacturing process, weldability, superelasticity, and high electrical conductivity, EGaIn/TPU composite fibers fabricated by wet spinning have considerable potential for flexible electronics.

Mushroom-mimetic 3D hierarchical architecture-based e-skin with high sensitivity and a wide sensing range for intelligent perception

Electronic skin (e-skin) is one of the most important components of future wearable electronic devices, whose sensing performances can be improved by constructing micropatterns on its sensitive layer. However, in traditional e-skins it is difficult to balance sensitivity and the pressure sensing range, and most micropatterns are generally prepared by some complex technologies. Herein, mushroom-mimetic micropatterns with 3D hierarchical architecture and an interdigital electrode are facilely prepared. The micropatterned sensitive layer is further developed through spraying carbon nanotube (CNT) dispersion on the thermoplastic polyurethane (TPU) film with mushroom-mimetic micropatterns (denoted as MMTC). Thanks to the "interlocking effect" between mushroom-mimetic micropatterns and the interdigital electrode in the as-prepared MMTC/interdigital electrode e-skin, the e-skin exhibits a high sensitivity (up to 600 kPa-1), a wide pressure sensing range (up to 150 kPa), a short response time (<20 ms) and excellent durability (15 000 cycles). The MMTC/interdigital electrode e-skin is capable of precisely monitoring health conditions via the as-acquired physiological parameters in real time. Moreover, such e-skins can be used to monitor gestures wirelessly, sense the trajectory of pressure stimuli and recognize Morse code under water. This study provides a cost-efficient, facile strategy to design e-skin for future-oriented wearable intelligent systems.

Hierarchical Fermat helix-structured electrochemical sensing fibers enable sweat capture and multi-biomarker monitoring

Wearable electrochemical sensors have shown potential for personal health monitoring due to their ability to detect biofluids non-invasively at the molecular level. Smart fibers with high flexibility and comfort are currently ideal for fabricating electrochemical sensors, but little research has focused on fluid transport at the human-machine interface, which is of great significance for continuous and stable monitoring and skin comfort. Here, we report an electrochemical sensing fiber with a special core-sheath structure, whose outer layer is wound by nanofibers with a hierarchical Fermat helix structure which has excellent moisture conductivity, and the inner layer is based on CNT fibers covered by three-dimensional reduced graphene oxide folds which have good sensing properties after modification of active materials such as enzymes and selective membranes. This kind of fiber enables efficient sweat capture, and thus only 0.1 μL of sweat is required to activate the device, and it responds very quickly (1.5 s). The fibers were further integrated into a garment to build a wireless sweat detection system, enabling stable monitoring of six physiological markers in sweat (glucose, lactate, Na+, K+, Ca2+, and pH). This work provides a feasible proposal for future personalized medicine and the construction of "smart sensing garments".

"Three-in-One" Multi-Scale Structural Design of Carbon Fiber-Based Composites for Personal Electromagnetic Protection and Thermal Management

Wearable devices with efficient thermal management and electromagnetic interference (EMI) shielding are highly desirable for improving human comfort and safety. Herein, a multifunctional wearable carbon fibers (CF) @ polyaniline (PANI) / silver nanowires (Ag NWs) composites with a "branch-trunk" interlocked micro/nanostructure were achieved through "three-in-one" multi-scale design. The reasonable assembly of the three kinds of one-dimensional (1D) materials can fully exert their excellent properties i.e., the superior flexibility of CF, the robustness of PANI, and the splendid conductivity of AgNWs. Consequently, the constructed flexible composite demonstrates enhanced mechanical properties with a tensile stress of 1.2 MPa, which was almost 6 times that of the original material. This is mainly attributed to the fact that the PNAI (branch) was firmly attached to the CF (trunk) through polydopamine (PDA), forming a robust interlocked structure. Meanwhile, the composite possesses excellent thermal insulation and heat preservation capacity owing to the synergistically low thermal conductivity and emissivity. More importantly, the conductive path of the composite established by the three 1D materials greatly improved its EMI shielding property and Joule heating performance at low applied voltage. This work paves the way for rational utilization of the intrinsic properties of 1D materials, as well as provides a promising strategy for designing wearable electromagnetic protection and thermal energy management devices.

Biocompatible Material-Based Flexible Biosensors: From Materials Design to Wearable/Implantable Devices and Integrated Sensing Systems

Human beings have a greater need to pursue life and manage personal or family health in the context of the rapid growth of artificial intelligence, big data, the Internet of Things, and 5G/6G technologies. The application of micro biosensing devices is crucial in connecting technology and personalized medicine. Here, the progress and current status from biocompatible inorganic materials to organic materials and composites are reviewed and the material-to-device processing is described. Next, the operating principles of pressure, chemical, optical, and temperature sensors are dissected and the application of these flexible biosensors in wearable/implantable devices is discussed. Different biosensing systems acting in vivo and in vitro, including signal communication and energy supply are then illustrated. The potential of in-sensor computing for applications in sensing systems is also discussed. Finally, some essential needs for commercial translation are highlighted and future opportunities for flexible biosensors are considered.

Conductive fibers for biomedical applications

Bioelectricity has been stated as a key factor in regulating cell activity and tissue function in electroactive tissues. Thus, various biomedical electronic constructs have been developed to interfere with cell behaviors to promote tissue regeneration, or to interface with cells or tissue/organ surfaces to acquire physiological status via electrical signals. Benefiting from the outstanding advantages of flexibility, structural diversity, customizable mechanical properties, and tunable distribution of conductive components, conductive fibers are able to avoid the damage-inducing mechanical mismatch between the construct and the biological environment, in return to ensure stable functioning of such constructs during physiological deformation. Herein, this review starts by presenting current fabrication technologies of conductive fibers including wet spinning, microfluidic spinning, electrospinning and 3D printing as well as surface modification on fibers and fiber assemblies. To provide an update on the biomedical applications of conductive fibers and fiber assemblies, we further elaborate conductive fibrous constructs utilized in tissue engineering and regeneration, implantable healthcare bioelectronics, and wearable healthcare bioelectronics. To conclude, current challenges and future perspectives of biomedical electronic constructs built by conductive fibers are discussed.

Toward a new generation of permeable skin electronics

Skin-mountable electronics are considered to be the future of the next generation of portable electronics, due to their softness and seamless integration with human skin. However, impermeable materials limit device comfort and reliability for long-term, continuous usage. The recent emergence of permeable skin-mountable electronics has attracted tremendous attention in the soft electronics field. Herein, we provide a comprehensive and systematic review of permeable skin-mountable electronics. Typical porous materials and structures are first highlighted, followed by discussion of important device properties. Then, we review the latest representative applications of breathable skin-mountable electronics, such as bioelectrical sensors, temperature sensors, humidity and hydration sensors, strain and pressure sensors, and energy harvesting and storage devices. Finally, a conclusion and future directions for permeable skin electronics are provided.

Carbon Quantum Dot/Chitosan-Derived Hydrogels with Photo-stress-pH Multiresponsiveness for Wearable Sensors

In recent years, hydrogels have attracted extensive attention in smart sensing owing to their biocompatibility and high elasticity. However, it is still a challenge to develop hydrogels with excellent multiple responsiveness for smart wearable sensors. In this paper, a facile synthesis of carbon quantum dots (CQDs)-doped cross-linked chitosan quaternary/carboxymethylcellulose hydrogels (CCCDs) is presented. Designing of dual network hydrogels decorated with CQDs provides abundant crosslinking and improves the mechanical properties of the hydrogels. The hydrogel-based strain sensor exhibits excellent sensitivity (gauge factor: 9.88), linearity (R² : 0.97), stretchable ability (stress: 0.67 MPa; strain: 404%), good cyclicity, and durability. The luminescent properties are endowed by the CQDs further broaden the application of hydrogels for realizing flexible electronics. More interestingly, the strain sensor based on CCCDs hydrogel demonstrates photo responsiveness (ΔR/R₀ ≈20%) and pH responsiveness (pH range ≈4-7) performance. CCCDs hydrogels can be used for gesture recognition and light sensing switch. As a proof-of-concept, a smart wearable sensor is designed for monitoring human activities and detecting pH variation in human sweat during exercise. This study reveals new possibilities for further applications in wearable health monitoring.

Miniaturized retractable thin-film sensor for wearable multifunctional respiratory monitoring

As extremely important physiological indicators, respiratory signals can often reflect or predict the depth and urgency of various diseases. However, designing a wearable respiratory monitoring system with convenience, excellent durability, and high precision is still an urgent challenge. Here, we designed an easy-fabricate, lightweight, and badge reel-like retractable self-powered sensor (RSPS) with high precision, sensitivity, and durability for continuous detection of important indicators such as respiratory rate, apnea, and respiratory ventilation. By using three groups of interdigital electrode structures with phase differences, combined with flexible printed circuit boards (FPCBs) processing technology, a miniature rotating thin-film triboelectric nanogenerator (RTF-TENG) was developed. Based on discrete sensing technology, the RSPS has a sensing resolution of 0.13 mm, sensitivity of 7 P·mm-1, and durability more than 1 million stretching cycles, with low hysteresis and excellent anti-environmental interference ability. Additionally, to demonstrate its wearability, real-time, and convenience of respiratory monitoring, a multifunctional wearable respiratory monitoring system (MWRMS) was designed. The MWRMS demonstrated in this study is expected to provide a new and practical strategy and technology for daily human respiratory monitoring and clinical diagnosis.

Electronic Supplementary Material

Supplementary material (additional figures and movies, including the production process of respiratory monitoring straps, the mechanical analysis of RSPS, RTF-TENG versus vector TENG sensors, the simulation studies of TE-TENG and FT-TENG, the additional characterization of RTF-TENG, the tensile and robustness tests of RSPS, the characterizations of the MWRMS during different sleeping positions, detailed circuit schematic of the MWRMS, the displacements and phase relations of RSPS, MWRMS for multifunctional respiratory monitoring) is available in the online version of this article at 10.1007/s12274-023-5420-1.

Transparent, Ultra-Stretching, Tough, Adhesive Carboxyethyl Chitin/Polyacrylamide Hydrogel Toward High-Performance Soft Electronics

To date, hydrogels have gained increasing attentions as a flexible conductive material in fabricating soft electronics. However, it remains a big challenge to integrate multiple functions into one gel that can be used widely under various conditions. Herein, a kind of multifunctional hydrogel with a combination of desirable characteristics, including remarkable transparency, high conductivity, ultra-stretchability, toughness, good fatigue resistance, and strong adhesive ability is presented, which was facilely fabricated through multiple noncovalent crosslinking strategy. The resultant versatile sensors are able to detect both weak and large deformations, which owns a low detection limit of 0.1% strain, high stretchability up to 1586%, ultrahigh sensitivity with a gauge factor up to 18.54, as well as wide pressure sensing range (0-600 kPa). Meanwhile, the fabrication of conductive hydrogel-based sensors is demonstrated for various soft electronic devices, including a flexible human-machine interactive system, the soft tactile switch, an integrated electronic skin for unprecedented nonplanar visualized pressure sensing, and the stretchable triboelectric nanogenerators with excellent biomechanical energy harvesting ability. This work opens up a simple route for multifunctional hydrogel and promises the practical application of soft and self-powered wearable electronics in various complex scenes.

Tough and Antifreezing MXene@Au Hydrogel for Low-Temperature Trimethylamine Gas Sensing

Trimethylamine (TMA) is one of the important chemical indexes to judge the freshness of marine fish. It is necessary to develop a low temperature TMA sensor to help the monitoring and prediction of the quality of marine fish in cold chain. Herein, a flexible low temperature TMA gas sensor featuring antifreezing and superior mechanical properties was developed based on the Au nanoparticle-modified MXene (MXene@Au) composite. MXene@Au was synthesized and then polymerized with a hydrogel composed of acrylamide (AM), N,N'-methylenebisacrylamide (BIS), sodium carboxymethyl cellulose (CMC), and EG, and the resultant MXene@Au hydrogel was found to exhibit excellent antifreezing performance even at extremely low temperature as well as high tensile strength, ultrastretchability, and toughness, which enabled an efficient gas sensing platform for TMA detection at low temperature. The TMA sensing properties of the flexible MXene@Au DN hydrogel sensor at 25 °C and a low temperature of 0 °C were studied, and a linear relationship between TMA sensitivity and concentration was built. The excellent sensing properties were maintained even under deformation. The application of the MXene@Au DN hydrogel sensor in detection of fish freshness at 0 °C was investigated. The result indicated the potential application of the flexible MXene@Au DN hydrogel gas sensor in dynamic quality monitoring and prediction of marine fish products during its transportation and storage in the cold chain.

High-Efficiency Electro/Solar-Driven Wearable Heater Tailored by Superelastic Hollow-Porous Polypyrrole/Polyurethane/Zirconium Carbide Fibers for Personal Cold Protection

With the frequent occurrence of extreme weather, using massive energy inputs to maintain the thermal stability of the indoor environment or the human body has become common, and such excessive overuse of nonrenewable energy has created numerous significant problems for modern society. Personal thermal management textiles which can provide the better thermal comfort with less energy consumption than the room heating devices have attracted vast attention in recent years. A polypyrrole/polyurethane/zirconium carbide (PU/PPy/ZrC) fiber with superior electrothermal/photothermal conversion was fabricated via a simple two-step strategy. The surface temperature of PU/PPy/ZrC fibers can reach 51.7 °C under IR lamp irradiation and 55.8 °C at 2 V. In addition, excellent electrical conductivity can be maintained even though the fiber has been stretched to 150%. Due to the porous and hollow structure of the PU/PPy/ZrC fiber, the fiber exhibits outstanding thermal stability and can reach a temperature difference of 5.2 °C. The excellent quick-drying properties allow for fast and complete drying of the material in both modes. Combined with the considerable mechanical properties and hydrophobicity of the PU/PPy/ZrC fiber, it demonstrates the outstanding potential and broad development of this dual-driven fiber for basic research and practical applications in personal cold protection.

A New Strategy to Fabricate Nanoporous Gold and Its Application in Photodetector

Nanoporous gold (NPG) plays an important role in high-performance electronic devices, including sensors, electrocatalysis, and energy storage systems. However, the traditional fabricating methods of NPG, dealloying technique or electrochemical reduction technique, usually require complex experimental procedures and sophisticated equipment. In this work, we reported a unique and simple method to prepare the NPG through a low-temperature solution process. More importantly, the structure of the NPG-based electrode can be further controlled by using the post-treatment process, such as thermal treatment and plasma treatment. Additionally, we also demonstrate the application of the resulting NPG electrodes in flexible photodetectors, which performs a higher sensitivity than common planar photodetectors. We believe that our work opens a possibility for the nanoporous metal in future electronics that is flexible, large scale, with facile fabrication, and low cost.

Flexible pressure sensors via engineering microstructures for wearable human-machine interaction and health monitoring applications

Flexible pressure sensors capable of transducing pressure stimuli into electrical signals have drawn extensive attention owing to their potential applications for human-machine interaction and healthcare monitoring. To meet these application demands, engineering microstructures in the pressure sensors are an efficient way to improve key sensing performances, such as sensitivity, linear sensing range, response time, hysteresis, and durability. In this review, we provide an overview of the recent advances in the fabrication and application of high-performance flexible pressure sensors via engineering microstructures. The implementation mechanisms and fabrication strategies of microstructures including micropatterned, porous, fiber-network, and multiple microstructures are systematically summarized. The applications of flexible pressure sensors with microstructures in the fields of wearable human-machine interaction, and ex vivo and in vivo healthcare monitoring are comprehensively discussed. Finally, the outlook and challenges in the future improvement of flexible pressure sensors toward practical applications are presented.

Multiscale Simulation on the Thermal Response of Woven Composites with Hollow Reinforcements

In this paper, we established a progressive multiscale model for a plain-woven composite with hollow microfibers and beads and investigated the general conductive thermal response. Micromechanic techniques were employed to predict the effective conductivity coefficients of the extracted representative volume elements (RVEs) at different scales, which were then transferred to higher scales for progressive homogenization. A structural RVE was finally established to study the influence of microscale parameters, such as phase volume fraction, the thickness of the fibers/beads, etc., on the effective and localized behavior of the composite system It was concluded that the volume fraction of the hollow glass beads (HGBs) and the thickness of the hollow fibers (HFs) had a significant effect on the effective thermal coefficients of the plain-woven composites. Furthermore, it was found that an increasing HGB volume fraction had a more significant effect in reducing the thermal conductivity of composite. The present simulations provide guidance to future experimental testing.

Ultrasensitive and highly stretchable fibers with dual conductive microstructural sheaths for human motion and micro vibration sensing

Conductive and stretchable fibers are important components of the increasingly popular wearable electronic devices as they meet the design requirements of excellent electrical conductivity, stretchability, and wearability. In this work, we developed a novel dual conductive-sheath fiber (DCSF) with a conductive sheath composed of a porous elastic conductive layer and cracked metal networks, thus achieving ultrahigh sensitivity under a large strain range. The core of the DCSF is made of thermoplastic polyurethane (TPU) elastic fiber wrapped in a porous stretchable conductive layer composed of carbon nanotubes (CNTs) and TPU. Next, a layer of gold film is deposited on the surface of the porous stretchable conductive layer by ion beam sputtering. Due to the fast response time of 184 ms and ultrahigh sensitivity in the 0-100% strain range (a gauge factor of 184.50 for a strain of 0-10%, 4.12 × 10⁵ for 10%-30%, and 2.80 × 10⁵ for 30%-100%) of the DCSF strain sensor, we successfully wove the fiber strain sensor into gloves and could realize the recognition of different hand gestures. Also the DCSF strain sensor can be applied to detect microvibrations efficiently. The demonstrated DCSF has potential applications in the development of smart wearable devices and micro vibration sensors.

Recent Advances in Multiresponsive Flexible Sensors towards E-skin: A Delicate Design for Versatile Sensing

Multiresponsive flexile sensors with strain, temperature, humidity, and other sensing abilities serving as real electronic skin (e-skin) have manifested great application potential in flexible electronics, artificial intelligence (AI), and Internet of Things (IoT). Although numerous flexible sensors with sole sensing function have already been reported since the concept of e-skin, that mimics the sensing features of human skin, was proposed about a decade ago, the ones with more sensing capacities as new emergences are urgently demanded. However, highly integrated and highly sensitive flexible sensors with multiresponsive functions are becoming a big thrust for the detection of human body motions, physiological signals (e.g., skin temperature, blood pressure, electrocardiograms (ECG), electromyograms (EMG), sweat, etc.) and environmental stimuli (e.g., light, magnetic field, volatile organic compounds (VOCs)), which are vital to real-time and all-round human health monitoring and management. Herein, this review summarizes the design, manufacturing, and application of multiresponsive flexible sensors and presents the future challenges of fabricating these sensors for the next-generation e-skin and wearable electronics.

Tunable and Nacre-Mimetic Multifunctional Electronic Skins for Highly Stretchable Contact-Noncontact Sensing

Electronic skins (e-skins) have attracted great attention for their applications in disease diagnostics, soft robots, and human-machine interaction. The integration of high sensitivity, low detection limit, large stretchability, and multiple stimulus response capacity into a single e-skin remains an enormous challenge. Herein, inspired by the structure of nacre, an ultra-stretchable and multifunctional e-skin with tunable strain detection range based on nacre-mimetic multi-layered silver nanowires /reduced graphene oxide /thermoplastic polyurethane mats is fabricated. The e-skin possesses extraordinary strain response performance with a tunable detection range (50 to 200% strain), an ultralow response limit (0.1% strain), a high sensitivity (gauge factor up to 1902.5), a fast response time (20 ms), and an excellent stability (stretching/releasing test of 11 000 cycles). These excellent response behaviors enable the e-skin to accurately monitor full-range human body motions. Additionally, the e-skin can detect relative humidity quickly and sensitively through a reversible physical adsorption/desorption of water vapor, and the assembled e-skin array exhibits excellent performance in noncontact sensing. The tunable and multifunctional e-skins show promising applications in motion monitoring and contact-noncontact human machine interaction.